Thermoelectric gas probe



R. S. BUCK THERMOELECTRIC GAS PROBE Filed Dec. 18, 1946 Feb. 7, 1950x/rl /l//////////////rlll Patented F eb. 7, 1950 THERMOELECTRIC GASPROBE lRichard S. Buck, Glastonbury, Conn., assigner to United AircraftCorporation, East Hartford, Conn., a corporation of Delaware ApplicationDecember 18, 1946, Serial No. '717,047

(Cl. 13G-4) Claims.

The present invention relates to a gas probe particularly adapted foruse in measuring the temperatures of hot gas streams.

One of the problems in measuring temperatures of hot gas is the loss ofheat from the thermocouple by radiation to the walls of the duct inwhich the hot gas stream is flowing. A cylindrical shield has been usedwithin which the thermocouple element is located with the shieldapproximating the temperature` of the gas and thereby reducing theamount of heat radiated by the thermocouple. The shield, however, inturn radiates heat to the walls of the duct and also loses heat byconduction along the supporting structure. A feature of this inventionis a shield by which to minimize the rate of heat transfer to the ductwalls both from theshieldv and from the thermocouple.

Another feature is an arrangement of' the supporting structure for thethermocoupleand shield by which to reduce further the loss of heat fromthe shield and from the thermooouple. A feature of the invention is aprobe which .may be eX- tremely small in size.

Other objects and advantages will be apparent from the specification andclaims, and from the accompanying drawing which illustrates anembodiment of the invention.

The single figure is a sectional View through the probe.

The probe consists of a substantially cylindrical shield I0 positionedwith its longitudinal axis substantially parallel to the gask streamrepresented by the arrows l2. The shield I0 is carried by a stem i4which may be releasably or permanently fastened in a support I6, thelatter in many cases forming an integral part of the wall I8 of the ductin which the gas is flowing.

The shield IU, which may be made, for example, from a stainless steel orfrom one of the nonferrous, high temperature alloys, is made up ofconcentric inner and outer walls 2D and 22 spaced from each other todenne a space 24 which may be evacuated, the ends of the walls 2U and 22being connected together at opposite ends to form the gas tight space24. Similarly the stem I4, which may be the same type of material as theshield l0, consists of spaced inner and outer walls 2S and 2B forming aspace 30 which may be evacuated and which intersects with the space 24.As shown the walls of the shield l D and the walls of the stem Ul mayall be integral, with a sealed connection where the inner wall 2Eintersects the wall 20 of the shield l.

The thermocouple element or junction 32 is 2 located within the shieldl0 and the connecting wires 34 and 36 of dissimilar materials whichsupport the thermocouple element may be located in a ceramic tube 38positioned within a metallic supporting tube 40 Within the stem I4, Asshown, the tube 40 may have projections 4t to hold it in spaced relationto the inner wall 2S of the stem in such a way that the-tubeA 40 withthe thermocouple Wires and element may be removed for replacement.

As shown, the probe is supported by engagement between the outer wallV2& 3 of the stem and the supporting element I6 se that the only heatconduction from the shield lll and the probe is along the outer wall 2 8to the suplJQrting-structure I6. By evacuating the spaces 24 and 3l] theheat transfer rate by conduction or connection between the inner andouter walls 2q and 2 2 of the shield or 26 and 28 of the stem ismaterially reduced, thereby maintaining the wall 2li of the shield at atemperature very close t@ the temperature of the gas passing through it.`For reducing the heat transfer further, the` outer surface of the innerwall 20 and the inner surface of the outer Wall ,2 2 may be covered.with ematerial of low emissivity 42 which, by reason of the absence ofoxygen in the space g4, retains its low emissivity without oxidizing atthe temperatures under which the probe is used, The space 24 may befilled with an inert gas if it is found that the vacuum therein. causesthe probe to collapse at high temperatures The inert ses will. preventOxidation of the .Coating of 10W emissivity..

With this arrangement not only does the gas probe have a double walledshield but the only way by which heat can travel by conduction from theinner shield is through the connection at the ends of the walls of theshield. Since the rate of conduction is materially reduced and since therate of radiation is reduced by the surfaces of low emissivity, whichmay be provided, for example,` by indium plating the Wall surfaces, asshown at 42, the temperature indicated by the thermocouple element willbe very close to the actual temperature of the gas stream.

Although the probe has been described as useful for measuring hightemperatures, it is also usable Where the gas temperature is lower thanthe temperature of the duct walls as in refrigerated gas. In this casethe shield prevents the thermocouple junction from receiving heat fromthe walls of the duct.

, It is to be understood that the invention is not limited to thespecific embodiment herein illustrated and described, but may be used inother one end of the tube to form a thermocouple junc- Q ways withoutdeparture from its spirit as dened by the following claims.

I claim:

1. A gas probe including a stem, thermocouple wires supported in saidstem and secured together 5 at their inner ends to form a junction, ametallic shield on the inner end of the stemV with the thermocouplejunction located'substantially centrally with the shield,y said shieldbeing open at rboth ends and having its axis at a substantial angle tothe stem, said shield having spaced walls connected together at opposite`ends to form a sealed chamber surrounding the thermocouple junction,the adjacent suri'as ofsaid walls having a coating of a material 'of lowemissivity for l5 reducing heat radiation.

`2. A gas probeincluding'a stem, `thermocouple wires supported in saidstem and secured together at their inner ends to Vform a junction, ashield on the inner end ofthe stem with the thermocouple `junctionlocated substantially centrally with the shield, said shield being openat both ends and havingitsaxis at a substantial angle to the s temsaidshield having spaced walls connected together at` opposite endsto form asealed i chamber surrounding the thermocouple junction, v

the adjacent surfaces of` said walls having a coatingjofia material oflow emissivity for reducing heat radiation, said chamber having theoxygen removed therefrom toprevent oxidation of thelocating.

3,. A gas probe `including a stem, thermocouple wires supported in saidstem and secured together f at their inner ends to form a junction, ametallic `shield on the inner end of the stem with the thermocouplejunction located substantially centifallywith theshield, said shieldbeing open at both` ends and having its axis at a substantial angle `to,thev stemfsaid shield having spaced connected together at opposite endsto formatie a" lsealed chamber surrounding the `thermocouple junction,the adjacent surfaces of said walls having a coating of a material oflowemissivity lfor reducing heat radiation, said chamber beingfiilledwith: an inert coating.

gas t'o prevent oxidation of the*` "1 4; A gas probe for a thermocoupleelement i'ncluding a pair Yof wiresof'dissimilar materials located in atube and Abeing connected together at4 tion, said probeincluding ahollow stem in which the thermocouple element is positioned with thejunction projecting irom'one end of the stem, a

double-walled substantially cylindrical metallic shield mounted on theend of the stem to surround the junction, said double-walled shieldhaving its inner and outer walls connected together adjacent oppositeends to form a chamber extending around the thermocouple junction, theinside wall surfaces oi the said chamber being coated with a material oflow emissivity.

5. A gas probe for a thermocouple element including a pair of wires ofdissimilar materials located in a tube and being connected together atone end of the tube to form a thermocouple junction, said probeincluding a hollow stem in which the thermocouple element is positionedwith the junction projecting from one end of the stem, a double-walledsubstantially cylindrical metallic shield mounted on the end of the stemto surround the junction, said double-walled shield having its inner andouter walls connected together adjacent opposite ends to form an annularchamber extending around the thermocouple junction, the inside wallsurfaces of the said chamber being coated with a material of lowemissivity, said chamber having the oxygen removed to prevent oxidationof the coating.

RICHARD S. BUCK.

REFERENCES CITED The following references are of record in the ille ofthis patent:

UNITED STATES PATENTS OTHER REFERENCES Franz, A., National AdvisoryCommittee on Aeronautics i Technical Memorandum No. 953, September 1940,page 5, 6 and 8.

King, W. J., Trans A. S. M. E. vol. (1943), page 426-7.

Revere, Instruments vol. 17 (1944) page 378.

